The present invention is a novel process for surface and subsurface treatment of crystalline materials to improve mechanical and material properties. Particularly, the present invention is concerned with a process to treat materials, such as metals and their alloys, by exposure to a high energy and power electron beam to restructure the metal or alloy grain structure to produce ultrafine grains. The ultrafine-grained metal or alloy exhibits improved mechanical properties including increased strength and hardness.
Ultrafine-grained crystalline materials are characterized broadly by grain sizes of 10-1000 nm and are often referred to as “nano-grained.” Materials with grain sizes within this region are known to possess enhanced strength over the same material with larger grain sizes. This phenomenon was discovered independently in the early 1950's by E. O. Hall [Proc. Phys. Soc., 1951, 64 (9), 747-753] and N.J. Petch and has come to be known as the Hall-Petch relation, which states that the yield strength of a material is inversely proportional to the grain size of that material. The increase in strength with decreasing grain size is due to the relaxation of stress from grain boundary dislocations. Thus, a higher applied stress is necessary to propagate dislocations through the material. This relation holds until the grain size reaches 10-100 nm at which point grain boundary sliding leads to reduction in yield strength with decreasing grain size. Therefore ultrafine-grained structuring of 10-100 nm grains demonstrates the highest yield strength potential.
Typically, tensile strength and ductility of a material are inversely related such that improving one will result in a reduction in the other. For ultrafine-grained material, the deformation mechanisms occur both through dislocation and grain-mediation, resulting in increased plasticity and potentially superplasticity. Experimental evidence has shown that the ductility can be enhanced if the nanostructure is induced through thermo-microstructural treatments as opposed to mechanical shearing while still producing a high tensile strength material [E. Ma, JOM, 2006, 58 (4), 49-53]. In addition, ultrafine-grained materials have exhibited enhanced formability and resistance to crack propagation.
The most common method for creating ultrafine-grained structuring within a metal alloy is to use high-shear mechanical methods. This includes severe plastic deformation, high-pressure torsion, surface mechanical attrition, high-energy milling, cryo-rolling, and sliding wear. However, these methods can be difficult to scale because of the extreme stresses that must be imparted to the metal alloy and, additionally result in decreased ductility. Due to the high degree of shear, the material must display some degree of plasticity, and therefore ceramics and brittle metals cannot be processed through known mechanical methods.
Another method of producing ultrafine-grained structure is through thermal treatment. This is typically conducted through annealing or tempering processes. For example, U.S. Pat. No. 3,178,324 details a thermal cycling process for inducing ultrafine-grained structuring in steel in which the body to be treated is heated in an oven or a melt bath to a specified temperature before being rapidly cooled. The immersion heating heats the surface of the body uniformly through heat conduction into the body of the material. Rapid cooling is achieved through forced air convection or liquid quenching to freeze grain growth. Due to the relatively even heating and cooling over the body surface, the grain structure is largely uniform. This method can, however, only treat near-net shape components as further machining, shaping, or tempering will likely disturb the grain structure.
The present invention is a novel process for creating ultrafine-grained structuring in solid materials such as metals, metal alloys, or ceramics with the purpose of improved material, mechanical, and/or thermal properties. One or more high energy electron beam(s) is/are used to locally heat the surface and subsurface of the body to be treated to above the liquidus transition temperature. We have recognized that this type of heating creates a localized melt pool which is then cooled by the surrounding bulk of the body which has not been heated by the electron beam(s). The surrounding, unheated body acts as a heat sink for the melt pool resulting in rapid cooling which freezes the ultrafine-grained structure and prohibits the grains from growing to thermodynamic equilibrium. The electron beam may raster over the surface of the body to process all or only specified sections of the body.
In electron beam processing, the processing depth directly scales with beam energy. Ultra-high energy (>1 MeV) has been achieved with superconducting linear electron accelerators (scLINACs). Beam energy, which is at least twenty times higher with scLINACs than other electron beam system, scales with process speed (heating rate). High-energy electron beams using a scLINAC are utilized to provide continuous electron beam exposure to the sample piece. The ability of the scLINAC electron beam to precisely deliver energy to a prescribed volume of material is unique as described in U.S. Pat. No. 9,328,976. As a way to succinctly illustrate the principles of the present invention and its advantages, the following discussion compares thermal material processing with electron beam scLINAC processing; specifically, with respect to the ultrafine-grained processing of metals. Processing of ultrafine-grained solids can be accomplished within the scope of our invention, however, with conventional conducting and scLINAC electron beam systems alike. Electron beams produced via scLINACs will be discussed going forward only as an exemplary case.
Energy delivery to a material via an scLINAC-produced electron beam is both rapid and efficient with nearly 100% of the electron energy being imparted to the material. In contrast, conventional thermal processing by contact, convection or irradiation heating is slow, and a large amount of energy is lost to the surroundings or to heat up the instrument itself. Laser irradiation generates plasma that reflects light. Plasma is also generated with such electron beam irradiation, but the electrons are transparent to plasma so that the thermal processing can continue uninhibited at higher power than laser heating. Lasers also only heat the subsurface via conduction, not volumetrically as electron beams heat. The volumetric heating is a result of the electrons penetrating the surface to instantly heat the material below the surface as opposed to heating the surface and relying on heat conduction in the way lasers transfer heat.
Our novel method of rapid heating and cooling differs drastically from known thermal processing techniques by using a point source to bombard the surface and subsurface of a material imparting heat through inelastic collisions. Only a particular area of the body at any one time is processed. Heat conduction can now be limited within the body to maximize the thermal gradients at the melt pool boundary. Flash melting has been demonstrated via high energy and power processing of metals where the impinging electron beam alone causes melting only in the region of intersection between the beam and the material. The timescale of the rastering beam is lower than that the timescale of thermal conduction leading to thermal gradients exceeding 1,000 K/mm. Cooling is provided by the surrounding body which is unaffected by the electron beam and acts as a heat sink. Due to the substantial thermal gradients, cooling rates exceeding 8,000 K/s are possible, leading to sub-cooling of the melt and grain structures frozen in the ultrafine size region.
Bulk thermal cycling, such as taught in above-mentioned U.S. Pat. No. 3,178,324, heats and cools the full bulk of the body through conduction and convection of the gas or liquid outside of the body being processed. In this known process, the goal is to minimize the thermal gradients within the material to ensure homogeneous grain structure formation. The thermal cycling involved in that known method does not melt the body but instead briefly raises it above solid-solid transition temperatures. Cooling is provided by a heat sink external to the processed body whereby the processed body is rapidly, physically moved from a heat bath to a heat sink external to the body unlike electron beam processing where large thermal gradients are required and the cooling comes from the body of the workpiece itself.
An object of the present invention is to employ electron beam processing to create ultrafine-grained structuring within solid materials, e.g., metal, metal alloys, semiconductor, ceramics, or composites to enhance material properties such as enhanced hardness, yield strength, tensile strength, toughness, formability, or resistance to crack propagation among others known to those skilled-in-the-art.
A further object of the present invention is to create hierarchical structuring in the body of the processed material with variations in the grain size as a function of position. Ultrafine-grained structuring through electron beam processing provides unique capabilities in manufacturing not achievable with other methods. The point source and rastering of the electron beam allows all or part of the body to be processed in a targeted manner. For example, the electron beam can be used to induce ultrafine-grained structuring only at the surface of body to improve the hardness of the surface but leave the underlying body untouched to take advantage of the untreated material properties in the bulk. In this way, layered or gradient structures can now be engineered to suit the material needs of an application. This may also be used to induce anisotropy into the body of a material when advantageous.
Yet another object of the present invention is to use electron beam processing to simultaneously additively manufacture components while creating ultrafine-grained structure or hierarchical structuring in the component. Our novel process may also be applied to creating ultrafine-grained structuring while additively manufacturing components to a near-net shape. Near-net shape is defined by the production of the component being very close to the final geometry such that little to no post-processing or refinement is necessary. Building up a component, layer-by-layer, our process for ultrafine-grained structuring may simultaneously be used to additively manufacture by, for example, fusing together raw feedstock materials such as sheet, wire, or powder while creating an ultrafine-grained structure within the component. The individual layers within the layer-by-layer manufacturing may also be processed under different conditions to have hierarchical structuring from the grain structure to the ultimate component structure. This may entail, again by way of example, alternating layers of ultrafine-grained and coarse-grained structures to take advantage of the material properties of each structure. It may be used to create a gradient in the grain structure in the direction of build.
The present invention greatly improves on the known variations of metallurgical microstructure annealing and tempering in that, by using electron beam technology, we are able to produce far-from-equilibrium ultrafine-grained structures by rapid and localized heating, followed by sub-cooling through large thermal gradients.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings and non-limiting examples herein.